1. Field of the Invention
The present invention relates to multilayer electronic components and methods for manufacturing the multilayer electronic components, and more particularly, to a multilayer electronic component including external electrodes formed on a laminate by plating and a method for manufacturing the multilayer electronic component.
2. Description of the Related Art
FIG. 9 shows a conventional multilayer electronic component 101, which is a typical laminated ceramic capacitor. The multilayer electronic component 101 usually includes a laminate 105, a first external electrode 108, and a second external electrode 109. The laminate 105 includes a plurality of insulating layers 102, a plurality of first internal electrodes 103, and a plurality of second internal electrodes 104. The first and second internal electrodes 103 and 104 are sheet-shaped and extend between the insulating layers 102. The laminate 105 has a first end surface 106 and a second end surface 107 opposed to the first end surface 106. The first and second external electrodes 108 and 109 are disposed on the first and second end surfaces 106 and 107, respectively. The first internal electrodes 103 have end portions which are exposed at the first end surface 106 and which are electrically connected to each other by the first external electrode 108. The second internal electrodes 104 have end portions which are exposed at the second end surface 107 and which are electrically connected to each other by the second external electrode 109.
In order to form the first and second external electrodes 108 and 109, paste electrode layers 110 are first formed such that a metal paste including a metal component and a glass component is applied on the first and second end surfaces 106 and 107 and then baked. First plating layers 111 primarily including nickel (Ni) are formed on one of the paste electrode layers 110. Second plating layers 112 primarily including tin (Sn) are formed on one of the first plating layers 111. Therefore, the first and second external electrode 108 and 109 have a three-layer structure including the paste electrode layer 110, the first plating layer 111, and the second plating layer 112.
The first and second external electrodes 108 and 109 are required to have good solder wettability because the multilayer electronic component 101 is mounted on a board by soldering. The first external electrode 108 is required to have a function of electrically connecting the first internal electrodes 103, which are electrically insulated from each other by the insulating layers 102, to each other. The second external electrode 109 is required to have a function of electrically connecting the second internal electrodes 104, which are electrically insulated from each other by the insulating layers 102, to each other. Therefore, the second plating layers 112 have good solder wettability and the paste electrode layers 110 have a function of electrically connecting the first and second internal electrodes 103 and 104. The first plating layers 111 have a function of preventing solder erosion.
The paste electrode layers 110 have a relatively large thickness of several ten to several hundred micrometers. In order to adjust the multilayer electronic component 101 to a predetermined size, the effective volume of the multilayer electronic component 101 that relates to capacitance must be reduced by the volume occupied by the paste electrode layers 110. The first and second plating layers 111 and 112 have a thickness of several micrometers. Therefore, if the first and second external electrodes 108 and 109 include the first and second plating layers 111 and 112 only, the multilayer electronic component 101 can have a larger effective volume and therefore have higher capacitance.
Japanese Unexamined Patent Application Publication No. 63-169014 discloses a method for forming external electrodes. In particular, the external electrodes are formed over both end surfaces of a laminate by electroless plating such that internal electrodes exposed at the end surfaces thereof are short-circuited.
In the method disclosed in Japanese Unexamined Patent Application Publication No. 63-169014, the catalytic activity of surfaces on which the external electrodes are formed is an important factor. In an example described in Japanese Unexamined Patent Application Publication No. 63-169014, the internal electrodes include a noble metal, such as palladium (Pd) or platinum (Pt), having high catalytic activity. This causes a significant increase in manufacturing cost.
When the internal electrodes do not include a noble metal, a catalytic metal, such as Pd, is applied to the surfaces. A process for applying the catalytic metal is very complicated. If no catalytic metal is applied to the surfaces, plating layers formed on the surfaces have low density, and therefore, the external electrodes have low reliability.
In order to enhance the soldering reliability of a multilayer electronic component, the external electrodes preferably extend to the edges of a principal surface and side surfaces of the laminate, which has a substantially rectangular parallelepiped shape, the principal and side surfaces thereof being adjacent to the end surfaces. The internal electrodes are not exposed at the principal and side surfaces. In order to allow the external electrodes to extend to the edges of the principal and side surfaces, plating layers for forming the external electrodes need to be grown from portions of the outermost internal electrodes that are exposed at the end surfaces. The amount of growth of the plating layers is large. Therefore, it is difficult to form the plating layers.
Japanese Unexamined Patent Application Publication No. 2004-146401 discloses a method for forming electroplating layers. In particular, the electroplating layers are formed by the following process: a conductive paste is applied onto edge portions of end surfaces of a laminate so as to be in contact with leading portions of internal electrodes, the edge portions extending in the direction in which internal electrodes are arranged; conductive layers are formed by firing or thermally curing the conductive paste; and the end surfaces thereof are electroplated. The electroplating layers are connected to the conductive layers.
According to the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-146401, external electrodes disposed on the end surfaces may have a small thickness. The electroplating layers can be uniformly grown due to the use of electroplating. Furthermore, the presence of the conductive layers, which are made from the conductive paste, allows the external electrodes to extend to edge portions of a principal surface and side surface of the laminate, the internal electrodes not being exposed at the principal surface and side surface, the edge portions being next to the end surfaces, and the laminate having a substantially rectangular parallelepiped shape.
Although the electroplating layers can be directly connected to exposed portions of the internal electrodes, conductive portions must be formed from the conductive paste prior to electroplating such that the leading portions of the internal electrodes are electrically connected to each other. A step of applying the conductive paste onto specific regions is complicated. The conductive portions have a relatively large thickness and therefore have a relatively large volume. This causes a problem in that the effective volume is reduced.
Since the conductive layers are made from the conductive paste, glass and/or organic components are created on the conductive layers in some cases. If the conductive layers having such components thereon are subjected to plating, plating defects may be caused.